Process devices, such as process fluid pressure transmitters, generally sense pressure using a pressure sensor coupled to at least one isolation diaphragm. The isolation diaphragm isolates the pressure sensor from process fluids that are being sensed. Process fluids, which can be highly corrosive, are thus kept isolated from the pressure sensor to avoid corrosion or damage to the pressure sensor. Pressure is transferred from the isolation diaphragm to the pressure sensor using a substantially incompressible isolation fluid in a passageway that fluidically couples the isolation diaphragm to a sensing diaphragm of the pressure sensor. The sensing diaphragm deflects in response to the applied pressure, and the deflection causes a change in an electrical parameter, such as capacitance, of a structure attached to or associated with the sensing diaphragm
The process fluid pressure transmitter is generally coupled to the process using a manifold or other suitable structure. The process fluid pressure transmitter is sealed to the manifold to ensure that process fluid does not leak. In a typical process fluid pressure transmitter, the sealing surface of the process fluid pressure transmitter contacts a non-metallic seal or other suitable structure. Welds for attaching the isolation diaphragm are sometimes located on the same surface that upon which the seal is seated. However, the non-metallic seal is compliant enough to tolerate variations in the surface of the weld and yet still effectively seal to the welded surface
Some non-metallic seals can retain pressures over 6000 psi. However, for higher temperature applications where the normal working pressure is over 6000 psi metal seals can provide some advantages. When using metal seals, for high pressure applications, the welds for attaching the isolation diaphragm cannot typically be located on the sealing surface due to specific surface finish requirements of the metal seals.
Providing a process isolation diaphragm assembly that retains all of the advantages of metal seals, with fewer drawbacks, would represent an improvement to process isolation diaphragm assemblies for high pressure process fluid applications.